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We analyze the chemical abundances of planetary nebulae and HII regions inthe M81 disk for insight on galactic evolution, and compare it with that ofother galaxies, including the Milky Way. We acquired Hectospec/MMT spectra of39 PNe and 20 HII regions, with 33 spectra viable for temperature and abundanceanalysis. Our PN observations represent the first PN spectra in M81 everpublished, while several HII region spectra have been published before,although without a direct electron temperature determination. We determineelemental abundances of helium, nitrogen, oxygen, neon, sulfur, and argon inPNe and HII regions, and determine their averages and radial gradients. Theaverage O/H ratio of PNe compared to that of the HII regions indicates ageneral oxygen enrichment in M81 in the last ~10 Gyr. The PN metallicitygradient in the disk of M81 is -0.055+-0.02 dex/kpc. Neon and sulfur in PNehave a radial distribution similar to that of oxygen, with similar gradientslopes. If we combine our HII sample with the one in the literature we find apossible mild evolution of the gradient slope, with results consistent withgradient steepening with time. Additional spectroscopy is needed to confirmthis trend. There are no Type I PNe in our M81 sample, consistently with theobservation of only the brightest bins of the PNLF, the galaxy metallicity, andthe evolution of post-AGB shells. Both the young and the old populations of M81disclose shallow but detectable negative radial metallicity gradient, whichcould be slightly steeper for the young population, thus not excluding a mildgradients steepening with the time since galaxy formation. During its evolutionM81 has been producing oxygen; its total oxygen enrichment exceeds that ofother nearby galaxies.

The population of planetary nebulae and HII regions in M81. A study of radial metallicity gradients and chemical evolution